Develop spinel structure and quantify phase transformation for nickel stabilization in electroplating sludge.

Nickel-laden electroplating sludge (Ni sludge) has always been a critical concern due to its potential hazards to the environment. This study proposed a strategy to stabilize nickel (Ni) via phase transformation into stable crystal structures through ceramic sintering. The Ni sludge was collected, and then fired with two ceramic precursors (α-Fe2O3 and γ-Al2O3) within a temperature range of 700-1400 °C for 5 h. After sintering scheme, phase identification was performed on the products, showing the NiFe2O4 and NiAl2O4 spinels as predominant Ni-hosting phases respectively in α-Fe2O3 and γ-Al2O3 series. Then, the Rietveld refinement was applied to quantify weight fractions of all phases (including crystal and amorphous phases), and the quantification results showed that the weight fractions of NiFe2O4 or NiAl2O4 spinels can reach around 87.7% and 83.1%, respectively in 1200 °C sintered products of both series. The transformation ratio (TR) of Ni was calculated as 99.9% and 99.7% accordingly, showing almost complete incorporation of Ni into the spinel structures. With a prolonged leaching procedure, the Ni stabilization effect after sintering was evaluated. The Ni leachability was dramatically decreased with the development of spinel structure under sintering processes, and the Ni leached ratio from the sintered products can reach lower than 0.06% even after 20-d prolonged leaching. Through this study, a promising and quantitative method was proposed for controllable Ni stabilization of the hazardous industrial sludge via developing spinel structures in the sintered products, which may provide a feasible strategy for the treatment and beneficial utilization of heavy metal-laden solid wastes.